Page 281 - Geothermal Energy Renewable Energy and The Environment

272 Geothermal Energy: Renewable Energy and the Environment

understood, the ability to accurately and precisely control fracture development and propagation
remains an area of active research and development.
Once fractures have been opened, it is important to maintain a reasonable aperture in order to
assure adequate fluid circulation. Proppants have been used in many oil and gas applications, but
they have proven problematic in EGS applications. Proppants require a fluid with controlled viscos-
ity in order to maintain them in suspension as fractures open and propagate. Gels of various types
have been used in other applications but their performance in EGS settings has been inadequate.
Research is currently underway to identify materials suitable for high temperature environments
that can be used as the carrier for proppants, as well as for the proppants themselves.
At the elevated temperatures of EGS systems, fluids will react with fracture surfaces that are
newly exposed. This reactive transport has the potential to modify fracture apertures, roughness,
and other properties. Some of these changes can be beneficial, such as dissolving minerals along the
fracture and thus increasing fracture apertures, or depositing stable minerals at asperities, thus more
securely propping open the fractures. However, the effects of reactive transport could also be det-
rimental, such as restricting fluid flow and decreasing permeability by depositing large volumes of
secondary minerals along flow paths, or causing unwanted fracture growth by corrosion. The ability
to model these processes has become quite sophisticated (Clement et al. 1998; Xu and Pruess 1998,
2001; Parkhurst and Appelo 1999; Glassley, Nitao, and Grant 2003; Phanikumar and McGuire
2004; Steefel, DePaolo, and Lichtner 2005). It is now possible to model three-dimensional hydro-
logical and geochemical processes at very high resolution, taking into account multiphase flow in
fractured, porous media. The results can assist with forecasting the short- and long-term interac-
tions that can be expected for injected fluids of different chemistries. Using this approach, it has
been suggested that the use of CO , rather than water, as the heat carrying fluid may be beneficial in
2
reducing chemical interactions (Pruess and Azaroual 2006).
Currently, the primary limitation of this modeling approach is the inability to obtain detailed
characterization of the in situ conditions that are the starting point for such simulations. Nevertheless,
this capability holds promise for providing the ability to develop detailed projections of the evolu-
tion of porosity, permeability, and geochemical and mineralogical properties in producing geother-
mal reservoirs.
reservoir management for sustainability
When undisturbed, geothermal reservoirs represent the natural evolution of a geological system
in which heat transfer is influenced by conduction and convection processes. The balance of these
two heat transfer mechanisms reflects the local geological and hydrological properties of the site.
In some hydrothermal systems, convective fluid movement plays an important role in establish-
ing the thermal regime. The geothermal regime at Long Valley Caldera (California), for example
(Chapter 6), is strongly affected by convective heat transfer. In other systems, particularly those
that are being considered as sites for EGS development, conduction is the dominant heat transfer
mechanism and convection plays a very minor role. When EGS development takes place in these
settings, convection-dominated heat transfer is introduced into the reservoir when the reservoir is
stimulated and fluid pumped through the fracture system. Since heat transfer by convection will
occur at a rate that is many times faster than the in situ conduction rate, heat will be removed faster
than it will be replaced. The challenge then becomes how best to manage the resource to assure that
it can sustain power production over the lifetime of the power generating facility. It also becomes
important to understand the time required for recovery of the system, in order to guide responsible
resource management.
Realistic modeling of the behavior of these systems requires the use of sophisticated simulation
tools that can account for fluid flow, chemical reactions, and heat transfer in complex three- dimensional
fracture networks. The reactive transport simulation tools referenced above are the best means for
accomplishing this. However, less computationally intensive approaches can be used to approximate
the behavior of these systems, and gain important insight into their behavior and evolution.